I had NTL Broadband isntalled on 26th June and have the same new modem MAC address problem. (apparently problem MAC addresses start with 00.14)
I kept trying to register multiple times a day with the same invalid modem message.
Tech support kept telling me to try again in 24-48 hours.
Today (Fri 30th) I phoned and asked when the servers were being updated and was told 3rd July. (was hoping it was today 15:30pm as someone suggested)
NTL have made me unhappy. (I called CS and logged a complaint)